This invention relates generally to an ultrasonic probe using a piezoelectric composite material, and more particularly to a multi-element ultrasonic probe forming a plurality of independently operable piezoelectric vibrators by disposing electrodes separately in a stripe form.
A typical known piezoelectric composite material 1 has a structure wherein a large number of piezoelectric ceramic rods such as lead zirconate titanate (hereinafter called "PZT") are arranged regularly in both X- and Y-directions with uniform spacings between them as depicted in FIG. 1 of the accompanying drawings. The structures of the principal portions of ultrasonic probes, which use such a piezoelectric composite material, are equipped with a plurality of electrodes (hereinafter called "stripe electrode elements") disposed in the stripe form on the surface of the piezoelectric composite material and enable the piezoelectric composite material at the portion corresponding to each stripe electrode to operate independently as a piezoelectric vibrator, are disclosed in Japanese Patent Laid-Open No. 220461983, U.S. patent specification No. 4,628,223. To reduce a side lobe, on the other hand, U.S. patent specification No. 4,658,176 discloses the arrangement wherein the pitch of the piezoelectric ceramic rods is smaller than the wavelength of the sonic wave of the fundamental resonance frequency and the width and pitch of the individual piezoelectric ceramic rods are sequentially different in the direction of their disposition. Incidentally, focusing mechanisms by phase adjustment of driving signals of the piezoelectric vibrators, and the like, are known in the art and hence, the description of such mechanisms is hereby omitted.
However, the prior art technique described above does not take into account the arrangement wherein the probe is constituted by use of a piezoelectric composite material having a high volume fraction for the piezoelectric ceramic rods. For, the piezoelectric composite material having a high volume fraction involves the problems that it has low flexibility because the occupying volume of organic materials that contribute to the flexibility of the piezoelectric composite material becomes small, the construction of a curved surface ultrasonic probe becomes therefore difficult, and since the occupying volume of the piezoelectric composite material is great, on the contrary, the spacing between channels becomes so small that cross-talk increases.
Japanese Patent Laid-Open No. 114239/1985 discloses a rectangular or ring-like electrode structure and U.S. patent application Ser. No. 661,928/'84 discloses an ultrasonic probe using a piezoelectric composite material whose volume fraction is from about 0.15 to about 0.75.
These prior art references do not consider, either, the drop of flexibility of the piezoelectric composite material in the same way as the prior art technique described already.
All the prior art techniques described above are merely based on the concept that when the volume fraction of the piezoelectric ceramic rod increases in the piezoelectric composite material, acoustic impedance of the piezoelectric material increases so that matching with water or a live body becomes difficult to establish. Therefore, they do not consider sufficiently a piezoelectric composite material having excellent vibration characteristics and an acoustic matching layer as the piezoelectric material for the probe.
On the other hand, U.S. patent application Ser. No. 944,523 discloses an input device using a piezoelectric composite material for computers, electronic typewriters, and the like, but this reference is different in the technical field and structure from the present invention and is therefore merely illustrative as a prior development.